7884079768

break, after 7 days ~5 % of the total core graphite is oxidized, and T(fuel)-max is about the same as with no air ingress. For the “chimney” case (double vessel break that allows air access to both the bottom and top of the core), and assuming a 2-meter high chimney “appears” above the vessel, air ingress flow is assumed to begin upon depressurization. The higher oxidation ratę than in the previous case eventually, after ~4 days, penetrates further up the core as the lower reflector and support structure are cooled to the point that little oxidation occurs there. T(fuel)-max is lower than in the reference - no air ingress - case, but the maximum vessel temperaturę is higher 453°C at 168 hr. With unlimited fresh air available, after 7 days ~10 % of the core graphite is oxidized. Some mitigating actions (to limit the air supply) are necessary.

4.4    P-LOFC with ATWS: In this PBMR design, recriticality occurs at about 28 hours, and T(fuel)-max reaches 2127°C at 103 hr. Maximum vessel temperatures are also higher, 711°C at 145 hr. Fuel failure after 7 days was 57%. Variations in this accident are sensitive to fuel and moderator temperature-reactivity feedback coefficients. As with the GT-MHR, if after recriticality the SCS is started (with still no scram), peak fuel temperatures would exceed limits even morę due to the selective undercooling.

4.5    D-LOFC with ATWS: Recriticality occurs at 31 hr. In this case, T(fuel)-max is 2166°C at 137 hr, and the maximum vessel temperaturę (496°C at time = 168 hr) was still rising slowly after a week. Fuel failure at the end of the week was 59%.

5.0 CONCLUSIONS

Both MHTGR designs show excellent accident prevention and mitigation capabilities even for well-beyond design-basis accidents due to their inherent passive safety features. The differences in the predicted absolute values of peak temperatures (for both fuel and vessel) for the two concepts for given accident scenarios should not be taken as definitive, sińce their finalized design features, such as vessel insulation strategies, have not been factored into the simulations. Other aspects of the predictions, such as assumed irradiated core thermal conductivities, temperature-reactivity feedback functions, and heat-sink related emissivities, are also dependent on many factors that should be considered in detail for specific design and operating conditions.

The value of sensitivity studies at this point (i.e., early) in the design and analysis is to provide estimates of the uncertainties in the predictions, and to guide further efforts in improving the design as well as the accuracy of the predictions. Clearly, the results for both concepts have shown the importance of effective core thermal conductivity functions and afterheat in the predictions of T(fuel)-max.

It was also shown, for the accidents postulated, that wide variations in the graphite oxidation ratę function multipliers did not significantly affect peak fuel temperatures, sińce the oxygen in the incoming air for the postulated buoyancy-driven air ingress accidents is typically depleted before it reaches the active core except for higher-flow, prolonged accident cases. Other considerations, however, such as predicting damage to hot structures that do encounter the oxygen, may reąuire additional refinement of the data and further analysis. It is very elear, however, that for long-term air ingress accidents, the availability of “fresh” air needs to be considered, and limited. Often overlooked is the fact that vessel-break accidents that could lead to such large-scale

13